Llanidloes (Welsh pronunciation: [ɬanˈɪdlɔɨs] ) is a town and community on the A470 and B4518 roads in Powys, within the historic county boundaries of Montgomeryshire (Welsh: Sir Drefaldwyn), Wales. The population in 2011 was 2,929, of whom 15% could speak Welsh. It is the third largest settlement in Montgomeryshire, after Newtown and Welshpool.
It is the first town on the River Severn (Welsh: Afon Hafren), counting from the source. The town's Member of Parliament is Steve Witherden of the Labour Party (MP since 2024) and its Member of the Senedd is Russell George of the Conservatives (MS since 2011).